Abstract
A number of investigators have observed localized heating by ultrasound near impedance discontinuities within tissues. It was hypothesized that mode conversion to shear waves at impedance discontinuities and subsequent absorption of these waves in a very small distance was the explanation for this heating. To test this hypothesis preliminary measurements of the shear acoustic properties of soft tissues at low megahertz frequencies were made. At these frequencies soft tissues are viscosity dominated, but the viscosities are orders of magnitude below those reported at lower frequencies. The low viscosities result in low velocities and extremely high absorption coefficients for shear waves in tissue. A mathematical model for mode conversion at a plane interface between two viscoelastic media was developed. Longitudinal and shear properties of tissue were then used to calculate the amount of mode conversion at tissue-air and tissue-water interfaces. It was found that negligible energy was converted to the shear mode.
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